The present invention relates to an optical unit, a backlight device, a liquid crystal module and a liquid crystal display apparatus, with improvements on luminance non-uniformity.
Liquid crystal display apparatuses have rapidly penetrated into markets, equipped with a larger display screen thanks to the recent advancement of technology. There is also a higher demand for a thinner frame so that liquid crystal display apparatuses can be installed anywhere.
Another higher demand is an improvement on luminance non-uniformity highly noticeable in larger display screens, mainly due to non-uniform luminance distribution of light of a backlight device that illuminates a liquid crystal display panel.
An improvement on luminance non-uniformity for a larger display screen is described, for example, in Japanese Un-examined Patent Publication No. 2008-3234 (referred to as Citation 1, hereinafter).
In Citation 1, a direct backlight device is installed just under or behind the back of a liquid crystal display panel, for illuminating the display panel.
A backlight device 101 of Citation 1 is equipped, as shown in FIG. 26, with light sources 102, a reflecting plate 103 and an optical unit 104. The light sources 102 are aligned on the reflecting plate 103.
The optical unit 104 consists of a light diffusing plate 105, a first light diffusing sheet 106, a light collecting sheet 107, a second light diffusing sheet 108, formed in this order from the light source's side. The first and second light diffusing sheets 106 and 108 have a transparent sheet base and a light diffusing surface formed on a light-emitting side of the sheet base. The light diffusing surface is applied with light diffusing particles. The light collecting sheet 107 is a prism sheet having a transparent sheet base and multiple prisms having a light collecting function, arranged on a light-emitting side of the sheet base.
The light components emitted from the light sources 102 are diffused by the light diffusing plate 105 and the first light diffusing sheet 106 and then collected by the light collecting sheet 107 while being diffracted in a direction orthogonal to the sheet 107. The collected light components are then emitted from the sheet 107, with enhanced luminance in a certain range of view angle. The light emitted from the sheet 107 is incident on the second light diffusing sheet 108 to undergo diffusion to have a lower level of luminance non-uniformity.
A thinner liquid crystal display apparatus requires a shorter distance H (referred to as optical-unit distance H, hereinafter) between the center of each light source 102 and the light diffusing plate 105, when it employs the backlight device 101 of Citation 1.
A typical optical-unit distance H is about 15 mm in known liquid crystal display apparatuses.
An experiment with the known backlight device 101 revealed that an optical-unit distance H shorter than 15 mm causes a bigger difference in luminance between the section just above (in front of) the light sources 102 and the other sections. Such a difference in luminance results in periodic luminance non-uniformity of illuminating light emitted from the backlight device 101, over a liquid crystal display panel.
One solution to such a problem is providing additional several optical sheets to the optical unit 104, which, however, causes cost-up and decrease in luminance.
Another solution to such a problem is providing more light sources 102 while maintaining sufficient luminance. Providing more light sources 102, however, leads to increase in circuit components in a driver for the liquid crystal display panel, such as an inverter, which results in cost-up and higher power consumption.
Further experiments with the known backlight device 101 revealed that the above two solutions improve uniformity of luminance for illuminating light emitted in a direction orthogonal to the backlight device 101 whereas do not improve uniformity of luminance sufficiently for illuminating light emitted in the other directions.
More optical sheets and/or light sources 102 are required to improve uniformity of luminance for illuminating light emitted in the other directions discussed above. An increase in number of the optical sheets causes decrease in total luminance. Moreover, an increase in the number of the light sources 102 leads to increase in circuit components, which results in cost-up and higher power consumption, as discussed above. Therefore, both are not practical solutions.